MOTION DAMPER FOR LARGE STRUCTURES

Abstract: An extrusion damper for interposing between two members to absorb energy of motion which may be induced between the two comprises an outer jacket (1) surrounding a body of a plastically deformable energy absorbing material (3) such as lead and a shaft (2) which moves through the outer jacket to deform the lead during induced motion, and means defining an extrusion orifice between the elements through which the energy absorbing material is forced during induced motion between the two to damp said motion. The body of energy absorbing material is subjected to approximately hydrostatic pressure preferably exceeding the shear yield stress of the energy absorbing material.

Description: MOTIONDAMPERFORLARGESTRUCTURES This invention relates to dampers of the type commonly referred to as extrusion dampers, used for reducing the effects of induced motion or displacement in a variety of structures and equipment. The dampers of the invention may be used in large structures such as bridges or buildings to reduce the effects of motion induced during earthquakes or from strong winds. They may also be used to damp motion of large or small moving objects. They may be used to damp motion in industrial machinery or engines or the like or from domestic appliances such as washing machines for example, or in any other application where it is desired to damp any motion, vibrations or similar. They may be used to damp displacement arising from thermal expansion. The dampers of the invention have various applications. BACKGROUND OF THE INVENTION Devices known as extrusion dampers which employ plastic deformation of certain materials to absorb energy are well known. US Patent No. 3,833,093 describes a form of extrusion damper consisting of an energy absorber material confined between an elongate outer jacket which is typically a cylinder, and a shaft which moves longitudinally within the jacket. The absorber material is typically lead while the jacket and shaft are typically formed of steel. Opposite ends of the jacket and shaft are connected between two members in a structure which are expected to move relative to one another, during an earthquake or other induced motion. A general discussion of these and related devices is given in "An Introduction to Seismic Isolation", R I Skinner, W H Robinson and G H McVerry, Wiley, 1993. Lead is the preferred deformable energy absorbing material for several reasons. First it yields at a room temperature shear stress of around 10.5MPa which is low compared with other metals and equivalent plastic materials. Second it restores its mechanical properties through recrystallisation and associated processes relatively rapidly following yield deformations, which provides outstanding resistance to work hardening under cyclic shear at ordinary temperatures. Third lead is readily available at the purity required to exhibit these properties. In practice such dampers are individually designed to protect a particular structure against damage by damping certain motions imparted to it. Their behaviour is quite closely approximated by that of an ideal Coulomb damper in having a force-displacement hysteresis loop which is nearly rectangular and practically rate independent over a wide range of frequencies. Research into performance of these devices is ongoing. SUMMARY OF THE INVENTION It is an object of the present invention to provide for improved performance of such dampers for seismic isolation and other applications. The invention broadly comprises a damper for interposing between two members to damp motion which may be induced between the two, comprising concentric elements each to be connected to a separate one of said members, a body of a plastically deformable energy absorbing material contained between said elements and subjected to approximately hydrostatic pressure at least approaching the shear yield stress of the energy absorbing material, and means defining an extrusion orifice between the elements through which the energy absorbing material is forced during induced motion between the two to damp said motion. Preferably the hydrostatic pressure applied to the energy absorbing material exceeds the shear yield stress of the material. Preferably the hydrostatic pressure is 5MPa or more and most preferably in the range 10-lOOMPa. Preferably the energy absorbing material is lead, but other energy absorbing materials which may be used include alloys of lead, aluminium at elevated temperature e.g. about 200°C, tin, zinc, brass, iron, super plastic alloys, or any other material having a low rate of work hardening, including also densely packed granular materials such as steel shot, glass beads, alumina, silica, silicon carbide or any other very hard granular material. In one form of damper of the invention the outer concentric element comprises an elongate outer jacket within which the body of energy absorbing material is fixed. The inner concentric element comprises a shaft and the means defining an orifice through which the energy absorbing material is forced during induced motion includes a part of the shaft of enlarged diameter. The shaft may optionally comprise a multiple number of parts of enlarged diameter along the length of the shaft to define a multiple number of said orifices through which the energy absorbing material is forced during induced motion. In another form of damper of the invention the outer concentric element comprises an elongate outer jacket and the inner concentric element comprises a shaft to which the body of energy absorbing material is fixed. The means defining an orifice through which the energy absorbing material is forced during induced motion includes a part of the outer jacket of reduced diameter across the interior of the outer jacket. Again, the outer jacket may comprise a multiple number of parts of reduced diameter across the interior of the outer jacket to define a multiple number of said orifices through which the energy absorbing material is forced. Dampers of the invention may be used in seismic isolation applications to damp seismic motion in large structures such as bridges or buildings or motion from very strong wind buffeting or similar. They may also be used in any other application where it is desired to damp any motion, vibrations, or similar. For example, dampers of the invention may be used to damp motion of engines or other industrial machinery. In domestic applications, dampers of the invention may be used in washing machines or spin dryers or dish washers to isolate vibrations. Small size dampers of the invention may be used as "microisolators" for sensitive electronic equipment such as the mechanism of a video recorder etc or in other similar applications. Numerous applications of the extrusion dampers of the invention are envisaged and the invention is not limited only to seismic isolation dampers. BRIEF DESCRIPTION OF THE DRAWINGS Preferred dampers of the invention will be described by way of example with reference to the following drawings, wherein: Fig. 1 is a perspective view of one form of damper of the invention. Fig. 2 shows the damper of Fig. 1 in longitudinal cross-section. Fig. 3 shows another form of damper of the invention similar to that of Figs 1 and 2 in longitudinal cross-section. Fig. 4 shows another form of damper of the invention also similar to that of Figs 1 and 2 in longitudinal cross- section. Fig. 5 shows a further form of damper of the invention again similar to that of Figs 1 and 2 in longitudinal cross- section. Fig. 6 shows another form of damper of the invention in longitudinal cross-section. Fig. 7 shows another form of damper of the invention similar to that of Fig. 6 in longitudinal cross-section, and Figs 8 and 9 are Mohr circle constructions which will be referred to further in description of the dampers of the inventio . DESCRIPTION OF PREFERRED FORMS The dampers shown in Figs 1 to 5 each comprise an outer jacket 1 which is typically formed of steel and may be cylindrical as shown, but could be of other cross-sectional shapes such as oval for example. A shaft 2 is able to move longitudinally through the outer jacket 1 in the direction of arrow A in each case. It is not necessary for the shaft to be positioned centrally within the outer jacket 1, but it could instead be off-set somewhat. The shaft 2 is also typically formed of steel. The shaft 2 and outer jacket 1 form concentric elements and the space between the outer jacket 1 and shaft 2 is filled with a plastically deformable material 3 such as lead so that in use the shaft moves through the lead. The shaft 2 moves through end cap 4 which also forms a bearing for one end of the shaft, and bearing 5 on the other side which floats. A tubular spacer 6 is provided between an end cap 7 at the other end of the outer jacket 1 and the floating bearing 5. A central part 2a of the shaft 2 has an enlarged diameter as shown. Fig. 1 shows the damper from the exterior while Figs 2 to 5 show the damper in longitudinal cross-section showing variations in construction which will be referred to further later. In use the outer jacket 1 of the damper is coupled to one member of a building or other structure through a suitable mechanical coupling, and the shaft is coupled to another member, through the end 2b of the shaft, which may move relative to the first in an induced motion. During such motion such as an earthquake in a seismic isolation application, the shaft 2 and in particular the enlarged diameter part 2a is forced through the material 3 such as lead. In effect the lead is forced through the orifice formed between the enlarged part 2a of the shaft 2 and the outer jacket. This creates a damping effect by conversion of kinetic energy to plastic deformational energy and heat, and further heat during re-crystallisation and other spontaneous recovery processes. In Figs 2 to 5 the shaft 2 comprises one part 2a of enlarged diameter but it is also possible for the shaft to comprise a multiple number of such enlargements, to increase the damping effect. In accordance with the invention the energy absorbing material 3 such as lead is prestressed under an approximately hydrostatic pressure at least approaching and preferably exceeding the shear yield stress of the material so that the material will always be in compression. With lead pressures of 5MPa or more, typically lOMPa to 30MPa, but also up to lOOMPa or more have been found effective. The effect of a hydrostatic pressure may be explained briefly by way of the Mohr circle constructions shown in Figs 8 and 9, which enables the properly tensor description of stress to be represented in two dimensions. A hydrostatic pressure applied to a body is then defined as one third the sum of the three principal stresses which act upon it. In Fig. 8 the hydrostatic pressure is 0, and the principal tensile stress σ x , the principal compressive stress σ y and the maximum shear stress σ' xy are all equal in magnitude. In Fig. 9 a hydrostatic pressure p equal to the shear stress '^ has been applied. The maximum tensile stress is then 0 so that the body is always under compression. Therefore the body cannot fail in tension. Returning to Figs 2 to 5 a number of alternative arrangements for applying hydrostatic pressure to the lead 3 (or other energy absorbing material) are shown. In Fig. 2 a block comprising layers of an elastomeric material 8 such as rubber and rigid material 9 such as steel is provided between the tubular spacer 6 and the end cap 7. In Fig. 3 a pad 10 of elastomeric material such as rubber is provided at either end of the absorber material 3. In both cases the length of the spacer 5 is such that when the end cap 4 is screwed home, the desired hydrostatic pressure is applied to the lead 3. In Fig. 4 the exterior of the body of absorber material 3 is surrounded by a sleeve 11 of elastomeric material such as rubber or reinforced rubber or other resilient material. In Fig. 5 the absorber material is entirely encased in such a layer of elastomeric material 12. Screw on end caps or multiple screw in plugs or hydraulically driven plugs which contact the lead at various points and apply pressure may alternatively be used. In each case the energy absorbing material 3 may be cast directly into place within the outer jacket 1 and around the shaft 2. In the embodiments of Figs 4 and 5 the rubber sleeve or casing may be stretched around the lead body after casting around the shaft and the shaft and lead then press fitted into the outer casing. As stated the damper may be cylindrical or alternatively oval, square, rectangular or any other desired shape in overall cross-sectional shape. The shaft and also other parts of the damper may optionally be coated with teflon, porcelain, titanium nitride, a hard ceramic material, glass, or similar. Preferably during assembly of the dampers the component parts are coated with a high temperature grease or other lubricant. Figs 6 and 7 show another type of damper of the invention. Again the damper comprises an outer jacket 21 typically formed of steel which is desirably cylindrical but could be of other cross-sectional shapes. In Fig. 6 one part of the outer jacket 21 is pressed to form a neck 21a of reduced diameter as shown. A shaft 22 which is enlarged at either end 22a passes through the outer jacket 21. The ends 22a of the shaft form plungers within the outer jacket 21. Seals 26 such as chevron seals may be provided to seal the plunger parts 22a of the shaft against the internal bore of the outer jacket. The plunger ends 22a of the shaft 22 are shown in cross-section. The end of the shaft extends out of the outer jacket on at least one side, to enable the shaft to be coupled into position in use. The shaft has a reduced diameter centre part 22b. In the drawings the shaft is shown as a three part shaft with the reduced diameter centre part 22b threading into the larger diameter parts 22a of the shaft at either end, as indicated at 22c. A body of lead 23 or other energy absorbing material is fixed around the reduced diameter part 22b of the shaft 22. The lead is subjected to hydrostatic pressure by screwing the plunger ends 22a onto the centre shaft part 22b to apply the desired pressure to the lead, which is also confined within the outer jacket 21. Other arrangements are possible. In use the outer jacket 21 is coupled to one member of a building or other structure, and one or both ends of the shaft 22 are attached to another member which " may move relative to the first in an induced motion. During such motion such as an earthquake in a seismic isolation application, the shaft 22 moves relative to the outer jacket 21, as it does so forcing the body of lead 23 through the orifice formed by the reduced diameter neck 21a in the outer jacket, creating a damping effect by conversion of kinetic energy to plastic deformational energy and heat, and further heat during re-crystallisation and other spontaneous recovery processes. Fig. 7 shows a damper similar to that of Fig. 6 except that the outer casing comprises a number of reduced diameter necks 21a in the outer jacket, to effectively define a multiple number of orifices through which the lead 3 is forced during induced motion, to increase the damping effect. Again the shaft and also the interior of the outer jacket may be coated with teflon, porcelain, titanium nitride etc and the component parts of the damper may be coated with a high temperature grease during assembly, as referred to previously. The foregoing describes the invention including various preferred forms thereof. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated herein as defined in the claims.

Claims: CLAIMS 1. A damper for interposing between two members to damp motion which may be induced between the two, comprising concentric elements each to be connected to a separate one of said members, a body of a plastically deformable energy absorbing material contained between said elements and subjected to approximately hydrostatic pressure at least approaching the shear yield stress of the energy absorbing material, and means defining an extrusion orifice between the elements through which the energy absorbing material is forced during induced motion between the two to damp said motion. 2. A damper according to claim 1, wherein the outer concentric element comprises an elongate jacket means within which the body of energy absorbing material is fixed and the inner concentric element comprises a shaft and wherein the means defining an orifice through which the energy absorbing material is forced during induced motion includes a part of the shaft of enlarged diameter. 3. A damper according to claim 1, wherein the outer concentric element comprises an elongate jacket means and the inner concentric element comprises a shaft to which the body of energy absorbing material is fixed, and wherein the means defining an orifice through which the energy absorbing material is forced during said induced motion includes a part of the elongate jacket means of reduced diameter across the interior of the jacket means. 4. A damper according to claim 3, wherein the elongate jacket means comprises a multiple number of parts of reduced diameter along the length of the jacket means to define a multiple number of said orifices through which the energy absorbing material is forced during said induced motion. 5. A damper according to any one of claims 1 to 4, wherein the body of energy absorbing material is subjected to hydrostatic pressure exceeding the shear yield stress of the energy absorbing material. 6. A damper according to any one of claims 1 to 4, wherein the body of energy absorbing material is subjected to hydrostatic pressure of lOMPa or more. 7. A damper according to any one of claims 1 to 4, wherein the body of energy absorbing material is subjected to hydrostatic pressure exceeding 20MPa. 8. A damper according to any one of claims 1 to 7, wherein the energy absorbing material is formed of lead. 9. A damper according to any one of claims 5 to 8 when dependent on claim 2, wherein means applying said hydrostatic pressure includes at least one elastomeric element. 10. A damper according to claim 2, wherein the shaft is coated with teflon, porcelane, titanium nitride, or other ceramic material, or glass. 11. A damper according to claim 3 or claim 4, wherein the interior of the outer jacket is coated with teflon, porcelane, titanium nitride, or other ceramic material, or glass. 12. A damper substantially as herein described with reference to any one or more of the accompanying drawings.

Inventors: ROBINSON WILLIAM HENRY, ROBINSON, WILLIAM, HENRY
Applicants: ROBINSON WILLIAM HENRY, ROBINSON, WILLIAM, HENRY
Application year: 1994
Main classification: F16F7
IPC codes: E04H9, F16F7, F16F9